Can you take a picture of where the ground is??
Not to insult you, as I don't know how much you know about audio and electronics, but is the ohm load correct for the amp?
When you say the amp gets hot, does it JUST get hot, or does it get hot enough to shut down?
Also, where are the gain and crossover settings set at?
How many watts is the amp?
Is the amp stable down to 1/2, 1, or 2 ohm?
As far as grounding out the amp, you want to try to find the most solid piece of metal available in the area, then clean any paint or oil off of the surface before you attach it.
With the seats, you said that they wouldn't come down.... if your amp ground is in the trunk, this should not affect the seats, might be a different problem.
One more thing, you said that your amp and deck are grounded out on the same bar... the deck or head unit should be grounded under the dash and the amp in the trunk.
We need a little more info to be able to properly diagnose the problem